Magnetic recording media are widely used as audio tapes, video tapes, computer tapes, computer discs and the like. A magnetic recording medium has become generally higher in density. The recording wavelength has become shorter and recording systems from an analog system to a digital system have been discussed.
Regarding the demand for higher density recording, a magnetic recording particulate medium formed by coating a dispersion of a ferromagnetic powder in a binder on a support has been inferior to a magnetic recording metal-film medium (metal-evaporated) in electromagnetic characteristics due to low packing density of a ferromagnetic powder. However, with the improvement of the performance of ferromagnetic powders and the advancement of the coating technique of an extremely thin layer in recent years, almost the same level of electromagnetic characteristics with those of the metal-film medium have been achieved. Further, a magnetic recording paticulate medium is superior in productivity and corrosion resistance.
As magnetic recording particulate media, those comprising a support having coated thereon a magnetic layer comprising a ferromagnetic powder, e.g., ferromagnetic iron oxide, Co-modified ferromagnetic iron oxide, CrO.sub.2, ferromagnetic metal (including alloys), etc., dispersed in a binder have been generally used.
For improving electromagnetic characteristics of a magnetic recording particulate medium, various methods such as means of improving magnetic characteristics of ferromagnetic powders or surface smoothness of magnetic layer have been suggested but these are not sufficiently satisfactory in view of higher density recording. Further, recording wavelength shows a tendency to become shorter with the progress of high density recording of a magnetic recording medium in recent years. The self demagnetization loss at recording time, i.e., output decreases due to increase of magnetic layer thickness, and the thickness loss at the time of reproduction have become serious problems. Therefore, a magnetic recording particulate medium comprising an extremely thin layer is proposed.
Further, as a video tape used for a magnetic tape cassette of Hi-8 or digital VCR for consumer use (SD specification) which is hereinafter referred to as DVC, a tape of magnetic recording metal-film medium in which a magnetic recording layer comprises a vaccum deposited metal (alloy), i.e., an ME (metal-evaporated) tape, has been nowadays put to practical use, and a system of using a magnetic recording particulate tape using a ferromagnetic metal powder, i.e., an MP (metal particulate) tape, and an ME tape concurrently has been realized.
For the coexistence with an ME tape, an MP tape must have higher output by thinning the magnetic layer, same as an ME tape, and also it is necessary that the relationship between the recording current and the reproduction output of an MP tape should be the same with that of an ME tape. If the recording current of an MP is increased gradually, the reproduction output thereof decreases due to recording demagnetization, but an ME tape does not have such a tendency and if the recording current is increased, the reproduction output tends to be saturated. Therefore, an Hi-8 deck takes the dual system of recording respectively with different recording currents for MP and ME tapes in practice, which complicates the circuit. For resolving this problem, it is necessary for the system to be capable of common use of an MP tape and an ME tape and recording is performed with the same recording current. However, there is a problem such that when an MP tape is recorded/reproduced with the optimal recording current of an ME tape, the output is lowered. On the other hand, when an ME tape is recorded/reproduced with the optimal recording current of an MP tape, an ME tape cannot exhibit its capability and the output is lowered. Accordingly, it has been necessary to make the optimal recording current of an MP tape almost equal to that of an ME tape.
Further, a signal of recording wavelength of 22 .mu.m has been adopted as a synchronizing signal and a signal of recording wavelength of 0.488 .mu.m has been adopted as a data signal in a digital video cassette recorder for consumer use. An overwriting erasure omitting an erasure head has been employed for the sake of making a recorder lightweight. For adopting overwriting erasure, a synchronizing signal is necessary to be erased by a data signal and the overwriting erasure factor is said to be preferably -20 dB or less. It is desired that an overwriting erasure factor be as low as possible as an essential characteristic for a magnetic recording medium.
For reducing an overwriting erasure factor, it has been thought that lowering the coercive force Hc of a magnetic layer would be sufficient. However, by the reduction of the coercive force Hc, an overwriting erasure factor can be improved but there is a limit because high frequency output is reduced due to recording demagnetization. Further, it has been suggested that the thickness of a magnetic layer be decreased for reducing an overwriting erasure factor. However, too thin a layer causes deficiency of magnetization amount. As a result output is entirely decreased whether it is short wave or long wave. Therefore, this can be limiting.
As a means for ensuring magnetization amount when a magnetic layer is extremely thin, there is a method of using a ferromagnetic powder having high saturation magnetization .sigma..sub.s and a method of decreasing the amount of a binder resin or the amount of nonmagnetic powders, such as an abrasive, etc. However, the dispersibility of a magnetic (coating) solution is reduced, the surface properties of a magnetic layer are degraded, or the strength of a magnetic layer becomes insufficient. As a result, running durability is extremely reduced.